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Double-sided organic displays for portable devices

"a nice achievement" was the response from colour display pioneer Cambridge Display Technologies to the launch of a transparent display for mobile phones as Eldis claimed "the world's first dual-emission organic light-emitting diode display (OLED) for a mobile phone."

It's not the first transparent (that is what double-sided OLED is) display, since Seiko Epson demonstrated a monochrome version, a couple of months ago.

"Colour is quite clever; when you're building a transparent display, the problem is to get enough of the activating thin film transistors into the substrate - without blocking the light," said CBT's chief executive, David Fyfe.

The purpose of building these organic LEDs is both cost, and battery life. But the battery gains only arise in the dark. In bright light, in the sun or under spotlights, the LEDs have to compete with the ambient light. They bleach out, as do ordinary back-lit thin film transistor (TFT) displays.

At that point, conventional displays have the option of going reflective, using the ambient light to bounce off a mirror behind the panel, or having a back-lit display. Transflective displays have a backlight, but are also reflective, working well in light or dark.

But the OLED is, potentially, far cheaper to make; and in the dim or dark light conditions, far more miserly with battery power.

Fyfe was less sure how significant the move was. "Their technology is Kodak technology; the Seiko Epson was monochrome only, using our technology," he said. "I didn't know they were doing it - looking at a transparent device. But that's something you can do with either version."

"The reason you can do this, is that the working part, the layer of light emitting pixels, is around 51 millionths of an inch thick; which allows you to make the cathode transparent, and get light out of both sides," said Fyfe.

Fyfe said the trick was possibly useful if people are looking at the device from both sides, but said that the application that would probably require this, would be when one display overlaid another. "I think the application that would make this useful, would be something like having one OLED film on top of a watch face, for example, so that a message could come up at a certain time."

It would also be useful for a "head up display" in an aircraft cockpit, allowing the pilot to see instruments on the windshield, without having to look down at a panel and take their eyes off the environment - and, potentially, in a car, suggested Fyfe.

"With Eldis, and the phone design, I'm not sure I see the point. If you look at the lid of the phone, the lid these days usually contains another low-power display, the sub-display, for telling you who the caller is before you flip the phone open and use power."

CDT recently announced a rationalisation, which reduced the number of factories and staff.

Photo courtesy Straits Times.